Piezoelectric crystal apparatus



July 14, 1942 J. M. woLFsKlLL 2,289,950

PIEZOELECTRIC CRYSTAL APPARATUS Filed April 28, 1941 2@ 26%@ /fM/Q;

J2; 70 2'? www Patented July 14, 1942 2,289,950 PIEzoELECTnIC CRYSTALAPPARATUS' John M. Wolfskll, Erie, Pa., assigner to Bliley ElectricCompany, Erie, Pa., a partnership composed of F. Dawson Bliley andCharles Coll- .man

Application April 28, 1941, Serial No. 390,830

15 Claims.

This invention relates to piezo-electric crystal apparatus in generaland more particularly to a method and apparatus of mounting crystals insuch a way that they are rigidly supported without having crystalcharacteristics affectedby the clamping or mounting means.

, This invention relates to a crystal holder of the general typedisclosed in my application Serial No. 343,468, Patent No. 2,240,453. I

An object of this invention is to provide a mounting for piezo-electriccrystals in which the crystal may be rigidly clamped without in any wayaffecting its piezo-electric activity.

Another object is to provide a piezo-electric crystal holder or mountingin which resonant clamping means are employed fr supporting the crystalrigidly.

Another object is to provide a mounting or holder for high frequencypiezo-electric crystals in which the crystal is rigidly clamped so thatits position will be maintained regardless of the vlbration of thecrystal or any slight jars or vibrations tc the holder proper.

A further object is' to provide a holder for piezoelectric crystals inwhich the mounting means also serve as the electrodes themselves, beingresonant to the crystal frequency.

In the prior art, the mounting of crystals in suitable electrodesthroughout certain frequency ranges has always presented a problem,primarily because the electrodes, when placed against the crystal face,abrade the crystal because of the v relative movement of the crystal tothe elec-l trodes. This movement, although very minute, is suiiicient tocause deleterious feects both to the crystal and the electrodes. Thereis a tendency for small amounts of quartz to be worn o and pieces ofmetal imbedded in the crystal faces. This naturally causes frequencychanges to the unit, and also tends to lower the Q or dampen thevibration of the crystal. This effect is cumulative and can over longperiods of time cause complete stoppage of oscillation of the crystah Inair-gap type holders, this eiect is eliminated except on the baseelectrode, on which the crystal rests, but heie the pressure is verylight, being- 050, Patent No. 2,240,451, uses a recessed electrode suchthat the crystal is in contact with the metal electrode only on the fourcorners. Other means of using recessed electrodes or clamping thecrystal only on the periphery of the electrode have been used, but inthese types of mountings the crystal portions in the clamped areasoscil- However, where the crystal is to be most crystals above about1500 kc. are used in late only very weakly, and the activity of theentire crystal unit is therefore somewhat affected by the clampingmeans.

My invention completely eliminates any abrasive action between thequartz and the electrode, because there is no relative movementtherebetween since the electrode is proportioned in such a Way as t0 beresonant at the crystal frequency or harmonic thereof, and thereforemoves with the crystal face. The activity of the crystal is also notaffected by the clamping or mounting means, because the electrode isresonant to the crystal frequency or harmonic thereof and no dampingaction'results from the clamping. The crystal is therefore rigidlysupported and clamped between two resonant members which are resonant tothe crystal frequency or a har` monic thereof. These resonant memberscan be used either on the rst, second, or third mode in order to get apractical proportion between the length and diameter The presentinvention uses resonant members to serve both as the supporting meansand as the electrodes for the crystal and has its main useulness in themounting of crystalsirom about 1500 kc., up to and includingr the highand ultra high frequencies.

Further details of this invention will be apparent to those skilled inthe `art to which it relates from the following specification, claimsand drawing in which, briefly,

Fig. 1 is an external perspective view of the crystal holder of myinvention;

Fig. 2 is a vertical sectional view through the crystal holder shown inFig. l;

Fig. 3 is a view along the line 3-3 of Fig. 2;

Fig. 4 is a view along the line 4 4 of Fig. 2;

Fig. 5 is a view of one of the connecting pins of the holder; i

Fig. 6 is a view of the electrodes and crystal 'employed for the purposeof explaining features of this invention;

Fig, '7 is an exaggerated view of a holder with convex electrodes; and

Fig. 8'is a view of a holder adapted for longitudinal mode plate typecrystals.

Referring to the drawing in detail, reference numeral I0 designates apiezoelectric crystal element clamped between the button electrodes Iland I2 which will be referred to as resonant buttons since the lengthsof these buttons in a direction normal to the clamped faces of thecrystal element are mechanically resonant or some multiple orsubmultiple of the crystal element frequency. The electrode II issoldered, welded, brazed or otherwise attached to the metal cover I3 ofthe crystal housing I4 which is f insulation material. The electrodes IIand I2 may be made of material such as stainless steel with the facesthereof contacting the crystal faces ground practically to a polish.

The bottom electrode I2 is attached as by soldering, welding, brazingand the like to the member I5 which is supported by the machine screwsI6 and II from the cover I3. The screws I6 and I1 are provided withporcelain, Isolantite or similar insulation bushings I8 and I9respectively between them and the cover I3 to prevent electrical shortcircuit of the crystal element. These screws are employed for thepurpose of clamping the crystal element and electrodes together and forthat purpose the screws are threaded into the plate member I5 althoughif desired these screws may pass through suitable holes of somewhatlarger size in the plate I5 and engage in threaded nuts on the otherside of said.

plate. In that case coil springs may be used between the nuts and theplate to add resilience to the electrode and crystal assembly.

The cover I3 is attached to the insulation housing I4 .by suitablemachine screws 20 which engage threaded nuts 2I embedded in theinsulation walls of the housing. One of these nuts 2I is attached to aconnecting member 22 shown in Fig.',5 and this is embedded in the baseof the housing for the purpose of forming electrical connection with thebase pin 23. The other y base pin 24 is connected to the connectingmember 25 partially embedded in the base and this in turn is connectedthrough the flexible lead 26 to the plate member I5. I

In Fig. 6 I have shown an exaggerated view of the manner in which thebutton electrodes I I and I2 vibrate in the shear mode. The amplitude ofthe vibration in one direction is shown in dotted lines and thisillustrates the shearing action of the vibration accomplished when thelengths of the electrodes in the direction normal to the faces of thecrystal contacted .by the electrodes, -are mechanically resonant tofthefrequency of the crystal or some multiple or submultiple thereof.

The formulas that apply for computing the length of the button requiredto be resonant at the specified crystal frequency are as follows. Thesehave been determined .from the fundamental considerations given inMechanical Vi-4 brations, by Den Hartog for a clamped free bar in whichthe resonant member acts as a cantilever, and these equations are givenbelow.

1.63T la F- Lf r) where E=Youngs modulus in dynes/sq. cm. D=Density orspecific gravity L==Length in cm.

T=Thickness of section 'in cm. F=Frequency in cycles/sec.

This equation is for a square or approximately square section in whichthe bending moment is taken about an axis at right angles to thethicknessr T. Substituting representative values for I steel of E andlD, E=20 1011 dynes/sq. cm. and D=8, we get For a round section,

.140d E F:7521/5 L=.211 cm.

For a crystal frequency of 10,000,000 cycles and the same diameterbutton of 1.27 cm.,

L=.0944 cm.

All of these equations are given for the first natural mode of vibrationof va cantilever. In

` some cases, it may be desirable to use a second or even a third mode4of vibration at the high frequencies to get a slightly longer oractually thicker resonant button electrode. ,Under conditions of secondmodc vibration, the constant in the frequency equation must bemultiplied approximately 6.25 times. The length of the pin vibrating inthe -second mode will then be increased by the square root of 6.25 or2.5 times. A second mode button then would be 2.5 times as long as afundamental button for the same frequency. This is of course assumingthe same diameter. The ythird or higher modes may be used in the samemanner. The length for the examples computed for the 2 megacycle crystalon the second mode would .be .527 cm., and the length for the 10 mc.crystal would be .236 cm.

Because of the high frequency at which this button type electrode isused and the consequentA small motion of the crystal, the relativelysmall ratio between the length of the electrode, or it actually might beconsidered thickness, to the diameter is of little consequence. It stilloperates as a cantilever or a clamped free bar. It has been foundnecessary at the lower frequency ranges where this type of electrode canbe employed to put a slightly convex face on the ends of the buttonelectrodes IIa and I2a which contact the crystal I 0a so that the motionis not impeded as shown in Fig. 7. The convexing need only be of theorder'of several' thousandths of an inch over the half inch or moredia-meter. As the frequency is increased, however, this is no longernecessary, and around 3 or 4 megacycles the end of the electrode orresonant member can be flat.

This type of holder is well adapted to the mounting of high frequencycrystals, and it has been checked experimentally throughout thefrequency range from `2 megacycles to 30 megacycles with excellentlresults. All of these holders assume the use of a plate type crystaloperating in a shear mode. If a longitudinal mode plate type crystal lc,such as an X-cut, is ernployed as shown in Fig. 8 the electrodes Hb and|2b and crystal I0c may be mounted in a resonant pin arrangement inwhich small resonant pins 2l are used to hold the electrodes at rightangles to the movement of said electrodes. The crystal 10c vibrates inthe direction indicated by the arrows so that the pins 21 engaging theedges of the electrodes Hb and I2b vibrate as canti'- levers withrespect to the firm supports 28. 1n this case it may be desirable toapply pressure to the electrodes I Ib and I2b. If these electrodes areplated or sprayed on the crystal faces the pins 2i may engage the edgesIc of the crystal if the crystal is of suicient thickness.

It has been found in actual practice that the length of the resonantmember is not critical and that it may vary within relatively widelimits because the pressure applied to the crystal to support it can berelatively high before affecting the crystal performance, andconsequently some ad-l justment in the resonant frequency of theelectrodes may be obtained by adjusting the pressure. The pressure canbe made so high that there is no need for any other means of supportingthe crystal or restraining the movement in the holder.

Various other modifications of this invention may be made withoutdeparting from the spirit and scope thereof and therefore I do notdesire to limit this invention to the exact details illustrated anddescribed, except as these details may be defined in the claims. v

What I claim is `as follows:

l. A piezoelectric crystal holder, comprising: a piezoelectric crystal,a pair of electrodes having faces for clamping said crystal facestherebetween each of said electrodes having a length in a directionnormal to said crystal faces dimensioned so as to be substantiallymechanicallyy resonant to a frequency of vibration of said piezoelectriccrystal. said `electrodes having a diameter parallel to the faces ofsaid crystal equal to several times the length thereof.

2. A piezoelectric crystal holder, comprising: a piezoelectric crystal,a pair of substantially dat electrodes for clamping portions of thefaces of said crystal therebetween, each of said electrodes having alength in a direction normal to said crystal faces dimensioned so thatsaid length is substantially mechanically resonant to a frequency ofvibration of said crystal, said electrodes having a diameter parallel tothe faces of said. crystal equal to several timesl the length thereof,and means for pressingsaid electrodes into clamping relation with thecorresponding faces of said crystal.

3. Piezoelectric crystal apparatus, comprising: a piezoelectric crystal,plug shaped electrodes each having a length in a direction normal tosaid crystal faces proportioned to vibrate with said crystal and adiameter equal to several times said length and means for clamping saidcrystal between said plug shaped electrodes.

4. Piezoelectric crystal apparatus, comprising:

a piezoelectric crystal, thin disc electrodes having faces engagingfaces of said crystal, the thickness of said disc electrodes beingproportioned to vibrate sympathetically with said a piezoelectriccrystal adapted to vibrate in the direction of its thickness, electrodesfor saidv crystal and the long dimensions of said disc electrodes beingequal to several times said thickness dimensions.

5. Piezoelectric crystal apparatus, comprising: a piezoelectric crystal,thin disc electrodes having faces engaging faces of said crystal, thethicknesses of said disc electrodes being proportioned to vibratesympathetically with said crystal and the long dimensions of said discelectrodes being equal to several times said thickness dimensions andmeans for clamping said crystal rmly between said disc electrodes,A

6. Piezoelectric crystal apparatus, comprising: a piezoelectric crystal,thin disc electrodes having faces engaging faces of said crystal, saidelectrode faces being slightly convex,l the thicknesses of said discelectrodes being proportioned to vibrate sympathetically with saidcrystal and the long dimensions of said disc `electrodes being equal toseveral times said thickness dimensions.

'1. Piezoelectric crystal apparatus, comprising:

- a. piezoelectric crystal, thin disc electrodes having faces engagingfaces of said crystal, said electrode faces being slightly convex, thethicknesses of said disc electrodes being proportioned to vibratesympathetically with said crystal and the long dimensions of said discelectrodes being equal to several times said thickness dimensions andmeans for clamping said crystal rmly between said slightly convex discelectrode faces.

8. Piezoelectric crystal apparatus, comprising:

crystal adapted to engage the faces of said crystal, and supportingmeans engaging the side edges of said electrodes, said supporting meansbeing proportioned to vibrate sympathetically withsaid crystal and saidelectrodes.

9. Piezoelectric crystal apparatus, comprising: a piezoelectric crystaladapted to vibrate in the direction oi its thickness, electrodes forsaid crystal, means engaging side edges of said electrodes for holdingsaid electrodes adjacent to faces of said crystal and for supportingsaid crystal, said last means being proportioned to vibratesympathetically with said crystal and said electrodes.

l0. Piezoelectric crystal apparatus, comprising: a piezoelectric crystaladapted to vibrate in the direction of its thickness, electrodes forsaid crystaly means for clamping said electrodes to faces of saidcrystal vincluding resonant pins mechanically resonant to a vibratingfrequency of said crystal engaging side edges of said electrodes.

ll. Piezoelectric crystal apparatus, comprising: a piezoelectric crystaladapted to vibrate in the direction of its thickness, electrodes for thefaces of said crystal, and pins'substantially mechanically resonant to avibrating frequency or harmonic or sub-harmonic of said crystal forclamp.a ing the side edges of at least one of said electrodes.

l2. Piezoelectric crystal apparatus, comprising: a high frequencypiezoelectric crystal, and means resonant substantially to the frequencyof oscillation of said piezoelectric crystal for supporting saidcrystal, said resonant means having areas substantially in contact withsaid piezoelectric crystal equal to or'greater than one-fourth the area'of an electrode face of said crystal sumcient to function as electrodesfor said crystal.

13. Piezoelectric crystal apparatus, comprising: a high frequencypiezoelectric crystal, and means resonant substantially to the frequencyof oscillation of said crystal for mechanically clamping and supportingsaid crystal, said resonant means having areas substantially in contactwith said piezoelectric crystal equal to or greater than one-fourth thearea of an electrode face of said crystal suflicient to function aselectrodes for said crystal.

14. Piezoelectric crystal apparatus, comprising: a high frequencypiezoelectric crystal, and means resonant substantially to a harmonic ofthe frequency of oscillation of said piezoelectric crystal forsupporting said crystal, said resonant means having areas substantiallyin contact with Said piezoelectric crystal equal to or greater thanonefourth the area of an electrode face of said crystal sufiicient tofunction as electrodes for said crystal.

15. Piezoelectric crystal apparatus, comprising: a high frequencypiezoelectric crystal, and means resonant substantially to a harmonic ofthe frequency of oscillation of said crystal for mechanically claxnpingand supporting said crystal, said resonant means having areassubstantially in contact with said piezoelectric crystal equal to orgreater than one-fourth the area of an electrode face of said crystalsuicient to function as electrodes for said crystal.

JOHN M. WOLFSKILL.

